Billions of years ago, Earth was purple in colour, sometimes even pink

Earth, the pale blue dot, as seen by Voyager 1 on 14 February 1990, wasn't always blue. Studies suggest that the earliest colour exuded by the oceans was purple, and even pink. This was between 3.5 and 2.4 billion years ago during the Archean eon. The theory was first proposed by Indian-American molecular biologist Shiladitya DasSarma, and has been termed the "Purple Earth Hypothesis." DasSarma theorised that early life on the planet did not work with chlorophyll but a molecule called retinal. It absorbed green and yellow light and reflected red and blue, which gave Earth the appearance of a purple planet. Some even say that the Earth's earliest colour was pink.

Retinal is still present around us and can be found in the plum-colored membrane of a photosynthetic microbe called halobacteria. This bacterium absorbs green light and reflects back red and violet light, so you see purple.

DasSarma says that primitive microbes that used retinal to harness the sun's energy dominated early Earth. So things on Earth at the time appeared purple. There were microbes that used chlorophyll but could not compete directly with those utilising retinal, DasSarma said. But they evolved the ability to absorb the wavelengths retinal did not use, helping them survive.

Also Read: Scientists discover something unexpected moving 2,700km under Earth's surface

"Chlorophyll was forced to make use of the blue and red light, since all the green light was absorbed by the purple membrane-containing organisms," William Sparks, an astronomer at the Space Telescope Science Institute (STScI) in Maryland, said. He helped DasSarma with his research.

A pink Earth

Today, the oceans are blue and plants green. But over 650 million years ago, cyanobacteria dominated Earth's oceans. Researchers have found the pink pigment in bacterial fossils from the Sahara Desert in Mauritania, West Africa. In fact, pink is considered the world's oldest-known colour. This pigment in the cyanobacteria living in the oceans gave the water a pinkish hue, making the "blue dot" a pink planet. According to findings published on July 9, 2018, in the journal Proceedings of the National Academy of Sciences, cyanobacteria are even older than algae.

Also Read: Astronomers find 128 new moons orbiting planet Saturn: 'Don't think Jupiter will ever catch up'

Ancient chlorophyll had been found fossilised in these bacteria. They survived on sunlight and spread everywhere on Earth. The fossilised chlorophyll has been discovered in concentrated form inside them. Scientists suggest that when they mixed with water, they released a pink colour. So everything, the water in the oceans and the land, likely appeared pink back then.

Organisms using chlorophyll and retinal survived together at one time. Over the years, chlorophyll won because it is more efficient than retinal. So today, Earth is green and the waters appear blue.